The present invention relates to fingerprint authentication and, more specifically, to a method and system for mapping a fingerprint image into a code such that the code resulting from fingerprint images taken from a same donor finger under varying environmental conditions exhibits invariance.
In order for fingerprint recognition to gain widespread acceptance as a reliable live-scan authentication tool, two major hurdles need to be overcome. The first involves obtaining a clear fingerprint image from a finger that may exhibit a variety of skin properties and the second involves obtaining high authentication accuracy based on fingerprint images.
Optical live-scan fingerprint scanners normally employ the mechanism of total internal reflection in a dark field, where the angle of observation from a camera is set at a so-called xe2x80x9cglass/air critical anglexe2x80x9d. In a two-dimensional picture, a transparent glass or plastic prism can be represented by an inverted, truncated triangle with the longer of two parallel surfaces being the platen used for making impressions of a live finger. Light illumination can come though the cut-off apex from below (i.e., the shorter parallel surface) or, alternatively, through one of the two nonparallel faces, with the camera pointing at the other nonparallel face.
Optical configurations such as the one described herein above are more rugged and produce images with higher resolution than most non-optical sensing methods such as capacitive, electric field and contact sensing. However, problems associated with so-called xe2x80x9ckeystone distortionxe2x80x9d and problems due to fingertips with dry skin have been identified as obstacles for its usefulness as a biometric device.
Keystone distortion can be minimized by the theory of tilted planes, in combination with a software equalizer algorithm performed on an expanded image bit map to trade image size for linearity. An optical solution is to incorporate geometric equalization to compensate for the differences in distance travelled by light rays, by introducing a complementary prism between the platen prism and the camera.
The so-called xe2x80x9cdry fingerxe2x80x9d problem is a more serious issue. In order to understand this problem, one can begin by examining what makes a print. Looking from a plan view, the skin surface of a fingertip consists of a number of curved ridges forming arches, loops and whorls, as well as composite and accidental flow patterns. Viewed in elevation under magnification, a segment of a length of ridge structure is made up of minute elevations (papillae) of uneven heights lining up in a row. This is why an inked impression of fingerprint shows broken lines and is subject to smudging and smearing of ink.
Pores exist only on ridges, and they are the opening of sweat glands under the epidermis. People who do not have dry skin problems usually have a deposit of sweat beads sitting on top of the pores. These liquid beads of sweat play a very critical role in latent print detection and render optical impression feasible. However, people with dry skin do not have as many sweat beads, which makes image taking difficult. The reason for this is now explained.
An image shows because there is intensity contrast amongst information-carrying pixels in the incident photons. If one considers the geometry of the prism as an inverted triangle, one can set up a dark field for observation purposes when the optical axis of the camera looking upwards from one side is aligned at the glass/air critical angle. Proper light illumination from underneath the prism will traverse through the transparent material of the prism and exit therethrough. Most of the ambient light surrounding the platen surface cannot reach the camera because reverse geometry of Snell""s law dictates that most of the photon energy entering the body of the prism from above will be confined to below the glass/air critical angle. Dark field photography can thus be made sensitive under a noisy environment.
U.S. Pat. No. 3,527,535 to John N. Monroe, uses the term xe2x80x9cdiffusely reflected lightxe2x80x9d to describe the mechanism whereby the points where the ridges touch will appear white (light) and the background and spaces between ridges will appear black (no light). This refers to a live-scan process in which a finger touches the prism platen to form an interface for optical fingerprint impression taking.
U.S. Pat. No. 5,233,404, issued on Aug. 3, 1993 to James H. Lougheed and Lam Ko Chau, suggested an improved mechanism for contrast enhancement to Monroe""s dark field principle. The term xe2x80x9cdispersed lightxe2x80x9d is used in their patent to account for the manifestation of ridges in contact with the prism platen as bright images observed at above the critical angle.
The term xe2x80x9cabsorbed/dispersed lightxe2x80x9d is also employed in U.S. Pat. No. 5,416,573, issued on May 16, 1995 to Thomas F. Sartor, Jr., to describe internally reflected light where the finger ridges contact the prism surface. This patent discloses making the observation angle of the detector sufficiently large so that the fingerprint image will be free of artifacts attributable to moisture.
However, U.S. Pat. No. 5,416,573 does not explain why such a diffusion or dispersion phenomenon is not observed by the detector at the same observation angle when a flat object is in contact with the prism platen. For example, if the angle of reflectance described in U.S. Pat. No. 5,416,573 has a range large enough to cover the angle of observation, then a two-dimensional image of a three-dimensional object having a flat surface at contact should appear in the dark field. However, this is not the case, as long as the object/platen boundary region is completely flat and air-tight. Further explanation becomes necessary when a clear and legible fingerprint ridge pattern of a reproducible nature is desired, which is the case when live authentication is being considered.
If one carves out a grid pattern on the flat surface of a solid object and places this carved flat surface in contact with the platen of a dark field scanner, the air-filled grid pattern would appear dark and the solid contact regions bright. For good contrast, it is necessary that rows and columns of air trapped in cavities be tight. What seems to happen to the camera mounted above the critical angle is the formation of contrast due to the following mechanism. Take the case where source light beam enters one of the two non-parallel surfaces of a glass prism. If the object is totally flat and devoid of air pockets, some light will be reflected and enter the camera aperture. The important phenomenon is that the reflected beam of light is homogeneous and uniform, as the solid/glass boundary layer is homogeneous and uniform. Therefore, no contrast is noticeable. With air pockets trapped in the flat object surface, the solid wall forming the air volume bounced the photon energy back in all directions, including a horizontal direction parallel to the platen surface. At this air/glass boundary layer, the reflected horizontal light enters the glass medium from an air medium, and will be confined after refraction through the optical path within the glass prism to an angle equal to or below the critical angle according to Snell""s law of total internal reflection in reverse. Consequently, less photon energy from the air/glass layer will enter the camera than that from the solid/glass layer due to differences in reflector geometry. It becomes clear that the degree of darkness in the dark image field is only relative, and it is the contrast ratio that will largely define the picture quality.
Live-scan devices make use of the difference in heights between a ridge and a valley of the fingertip skin surface. A ridge is typically tens of microns (one millionth of a meter) higher than a valley on a free standing up-turned finger. Upon turning the fingertip 180 degrees and being pressed against a platen, the difference in heights is reduced somewhat depending on the amount of applied pressure, but is still ten or more times larger than the thickness of layer of sweat which is typically a few microns. Papillary ridges originate from the minute elevations (papillae) of the epidermis, and are of uneven heights when viewed under a microscope. In absence of sweat moisture, some of the shorter ridges will fail to trap air pockets and become indistinguishable with valleys and are therefore xe2x80x9cmissedxe2x80x9d. This faint impression phenomenon is equivalent to the under-inked situation and is usually attributed to the xe2x80x9cdry fingerxe2x80x9d problem. Because air/glass boundary layers are ill defined in a hard, dry finger, the contrast ratio is poor compared with a soft, moist finger.
Since sweat pores run in single rows along the ridges only, and human sweat is made of water and lipid having a higher refractive index than air, a non-dry fingertip in contact with a platen has two refractive media instead of just air, namely, both liquid and air. It is a very fortunate fact that valleys do not contain sweat pores so that they can be separated from ridges through refractive discrimination. For the glass/liquid/air configuration of live-scan imaging, the liquid/glass boundary layer serves to raise the critical angle above that of the air/glass angle resulting in more photon energy incident to the camera aperture. When the level of liquid is high enough to touch the shorter ridges, but not high enough to fill up the air cavities of the valleys, the amounts of photons entering the dark fields is largest from the tall ridges, medium from the short ridges, and smallest from the partially air-filled valleys, respectively. However, all three objects are distinct in the dark image plane representing an optimum condition of operation. When the level of liquid fills up the air cavities, the overall liquid/glass boundary layer becomes homogeneous and uniform, resulting in an over-inked equivalent situation where the three objects become indistinct and result in total contrast destruction. This information deletion is worse than the under-inked condition, and is the subject of U.S. Pat. No. 5,416,573.
The remedy for dry fingers is to apply a thin layer of liquid externally between the object and the glass platen. It is possible to select a liquid with special optical properties so that an optimized contrast ratio between ridges and valleys can be achieved by chemical, optical and photon-sensing means. The above description demonstrates the importance of the thickness of liquid, almost analogous to the ink impression taking process. Once this remedy is implemented, and it is well known that the ridge pattern stays stable when applied pressure exceeds a certain limit, a clear and legible fingerprint can be captured, which should be reproducible, thereby rendering live fingerprint authentication technology feasible.
Assuming that a satisfactory image can be taken using one of the above-described (or other) methods, the second problem, i.e., that of reaching a high authentication accuracy, must be solved before live-scan fingerprint authentication can be considered a reliable tool in the fields of law enforcement, resource protection, electronic or non-electronic commerce, etc.
Two factors that limit the capability of live-scan devices to achieve high authentication accuracy are the rate of false acceptance and the rate of false rejection. The false acceptance rate is the frequency with which a device would accept the fingerprint of a donor during a matching operation between a query print from the donor and a different donor""s pre-stored template. The false rejection rate is the frequency with which a device would reject the fingerprint of a donor during a matching operation between a query print from the donor and the donor""s own pre-stored template. Solving the problem of achieving a high authentication accuracy therefore reduces to keeping the rates of false acceptance and false rejection to within acceptable upper bounds.
A general approach to reducing the false acceptance rate is to seek out features that are unique amongst fingerprint images from different donors. Much research has been done in this area over the last century, with computerized matching techniques having been introduced more recently. However, despite enormous investments by governments and corporations, fingerprint authentication has still not become the choice identification technology for most high-reliability applications. A reason for this is that many of the authentication methods that are capable of lowering the false acceptance rate actually suffer from a high false rejection rate. In other words, existing fingerprint authentication methods are often incapable of recognizing that certain features, which appear to be different between a query image and a pre-stored template, may in fact originate from the same finger.
A general approach to reducing the false rejection rate is to seek out features of a fingerprint image that do not vary when images are taken from the same donor finger. However, this requirement is difficult to satisfy, as a given donor""s finger will likely have a different orientation, humidity, temperature and/or pressure at the time when a query print is taken, compared to the time at which the pre-stored template was taken. Moreover, there is an added difficulty of ensuring that improvements in terms of lowering the false rejection rate do not cause the false acceptance rate to increase beyond an acceptable level.
Currently, there are no approaches to live-scan fingerprint authentication that can provide both a false acceptance rate and a false rejection rate that are sufficiently low for the technique to be used in high-reliability or high-security applications.
The present invention recognizes that a frame of reference for a fingerprint image can be defined on the basis of the features of the fingerprint image itself, which eliminates the requirement for defining an orientation with respect to an external reference frame such as the platen on which the finger is placed. Moreover, the present invention recognizes that even though the temperature and humidity of a finger may be different when the image is taken at different times, certain features of the fingerprint image, when measured with respect to the frame of reference, will exhibit merely minor variations. Upon quantization, there results a metric that is invariant from one fingerprint image to the next, for the same finger, thereby reducing the potential of a false rejection.
Moreover, robustness of the metric carries through, even when the metric is formed of a plurality of sub-metrics defined for respective sectors of the fingerprint image. It has been found that when the number of sectors is sufficiently large, yielding a corresponding number of sub-metrics assembled together in forming the metric, the probability of assigning the same metric to two different fingers can be reduced to an acceptable level. This permits a reduction in the false acceptance rate, in conjunction with the above mentioned reduction in the false rejection rate.
The invention may therefore be summarized according to a first broad aspect as a method, system and/or computer readable-storage medium for generating an identification code from a fingerprint image exhibiting a plurality of features. The method includes establishing a reference point from the plurality of features, generating at least one metric from the reference point and at least one of the plurality of features and forming the identification code from the at least one metric.
According to a second broad aspect, the invention may be summarized as a method and/or system for generating an identification code for association with a human finger. The method includes obtaining a fingerprint image of the finger, identifying a plurality of features on the fingerprint image, establishing a reference point from the plurality of features, generating at least one metric from the reference point and at least one of the plurality of features, and forming the identification code from the at least one metric.
The invention may also be summarized according to a third broad aspect as a method and/or system for generating an identification code for association with a human finger. The method includes obtaining a plurality of fingerprint images of the finger, identifying, for each fingerprint image, a respective plurality of features, establishing, from each plurality of features, a respective reference point, generating, from each plurality of features and the respective reference point, a respective metric, identifying a most frequently occurring metric from amongst the generated metrics, and forming the identification code as a function of the most frequently occurring metric.
The invention may be summarized according to a further broad aspect as a signal embodied in a transmission medium, comprising a code associated with a human finger, the code including at least one quantized metric generated as a function of (i) at least one of a plurality of features and (ii) a reference point established from the plurality of features.
According to yet another broad aspect, the invention may be summarized as a fingerprint-based user authentication system. The system includes a database of identification codes uniquely associated with respective users, an input device for obtaining, from a test user, a first fingerprint image and information identifying a first user and a controller connected to the input device and to the database. The controller is operable to access the database in order to obtain a first identification code associated with the first user, process the first fingerprint image in order to generate a second identification code and authenticate the test user as the first user by comparing the first and second identification codes to one another.
These and other aspects and features of the present invention will now become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying drawings.